Energy dependence of particle ratio fluctuations in centralPb+Pbcollisions fromsNN

Abstract

We present measurements of the energy dependence of event-by-event fluctuations in the K/π and (p +p)/π multiplicity ratios in heavy-ion collisions at the CERN SPS. The particle ratio fluctuations were obtained for central Pb + Pb collisions at five collision energies, √ s NN , between 6.3 and 17.3 GeV. After accounting for the effects of finite-number statistics and detector resolution, we extract the strength of nonstatistical fluctuations at each energy. For the K/π ratio, larger fluctuations than expected for independent particle production are found at all collision energies. The fluctuations in the (p +p)/π ratio are smaller than expectations from independent particle production, indicating correlated pion and proton production from resonance decays. For both ratios, the deviation from purely statistical fluctuations shows an increase toward lower collision energies. The results are compared to transport model calculations, which fail to describe the energy dependence of the K/π ratio fluctuations.

System-size dependence of particle-ratio fluctuations in Pb + Pb collisions at 158AGeV

“…III C and [18]). Except for the trigger and beam intensity the experimental conditions in this analysis are the same as described in [9,10]. The raw data were recorded in 2000 with a low intensity Pb beam of typical intensity of 10 4 ions/s in order to minimise pileup and δ-ray backgrounds in the TPCs.…”

“…The loose and tight sets of track cuts used in the present analysis are given in Table I. These are identical to those employed previously in NA49 analyses of fluctuations [9]. The acceptance is not only determined by the track selection cuts.…”

“…More details about the employed MLM and a description of extensive tests validating the statistical identification procedure can be found in [9,22].…”

“…In addition the multiplicity distribution of the mixed events was constructed to be the same as the corresponding distribution of the real events. As described in detail in [9], the mixed events contain no correlations due to physical processes, but effects from finite number statistics remain. The measured dE/dx information is still attached to the individual particles, so that the likelihood method can be applied to the mixed events in the same way as to the original events.…”

“…This motivated the analysis of event-byevent fluctuations of the (K + + K − )/(π + + π − ) (denoted K/π) [9], (p +p)/(π + + π − ) (denoted p/π) [9], and (K + + K − )/(p +p) (denoted K/p) ratios [10] as function of the center-of-mass energy by means of the observable σ dyn [see Eqs. (4) and (5) in Sec.…”